Plans by the Scottish government to get more women into work by providing free childcare involve "wishful thinking" in their costing, the Treasury has said.
The SNP has pledged to provide all three and four-year-olds, and vulnerable two-year-olds, with 1,140 hours of childcare per year by 2020.
It estimates that the policy could result in about 104,000 women entering employment and an additional £700 million in tax revenue which would help pay for the additional childcare.
But Treasury analysts said of the mothers affect by the policy only around 83,000 are not employed.
– Treasury analysts
Even if every mother out of work moved into work - in itself highly unlikely - there would still be a shortfall of 21,000.
A spokesman for Scotland's Finance Secretary John Swinney said:
– A spokesman for Scotland's Finance Secretary John Swinney
In terms of our childcare commitments and boosting the number of women in work, it is our commitment to stop wasting money on Trident and on contributing to the running costs of Westminster that gives us the ability to invest in these other priorities.